The object of this research is to study the interaction of embryonic murine neurons which have been totally dissociated from one another and then allowed to aggregate with one another in cell culture using the gyrorotatory system of Garber and Moscona. In particular, cells which can be identified on the basis of their morphology or their neurotransmitter content are cultured in the presence and absence of neighboring cells and target cells to which they normally project. The consequences of these cell-cell interactions are determined both at the light and electron microscopic level, as well as neurochemically. Thus, the differentiation of these cells and the factors governing such differentiation can be studied under well defined conditions.